Transfer molding of thermo setting plastics is a well known and commonly used technique for packaging an integrated circuit (IC). The transfer molding process molds a liquefied plastic material around portions of an IC. A transfer mold is placed over the IC and the transfer mold is filled with liquefied thermosetting plastic material. After the liquefied thermosetting plastic material has been compacted and solidified, the transfer mold is removed from the IC. The solidified plastic material provides mechanical and environmental protection for the encapsulated IC. An exemplary transfer molding process will be more fully described with reference to FIGS. 1 though 5.
Advances in molding technology have provided a number of methods for producing openings in plastic parts that expose selected areas of an integrated circuit chip. These openings are essential in applications where the functionality of the integrated circuit device relies on having physical, environmental or optical access directly to the surface of the integrated circuit. Examples of devices that need such access include, without limitation, cameras that use CMOS chips and CCD chips, pressure transducers, and biometric sensors that rely on capacitative coupling (e.g., for the detection of finger prints).
There are two basic types of transfer molding techniques for producing exposed IC surfaces. In both types it is necessary to exclude plastic from selected areas of the IC surface. To do this it is necessary to contact or clamp the delicate IC die surface with part of the transfer mold. However, transfer molds are usually made of hardened steel so that any surface of the transfer mold that makes contact with the delicate IC die surface is firm and unyielding. The transfer mold does not accommodate the normal dimensional variations that exist in the height and location of the IC die surface. This means that when the transfer mold contacts the IC die surface, the transfer mold usually causes damage to the IC die surface.
A first type of transfer molding technique addresses this problem by providing a compliant material in those areas where the transfer mold contacts the IC die surface. The compliant material is semi-permanently attached to the face of the transfer mold where the transfer mold contacts the IC die surface. The compliant material cushions the clamping action of the transfer mold against the IC die surface.
For example, cushions made of silicone can be attached to the transfer mold in the critical contact areas where the transfer mold contacts the IC die surface. See, for example, U.S. Pat. No. 5,897,338. This type of technique is referred to as the “soft insert” molding technique. There are some problems, however, with the soft insert molding technique. Silicone is a soft material and wears rapidly. It must be replaced often. Replacement of the silicone material is a relatively expensive task. In addition, the soft silicone material can collect embedded particles over time. These embedded particles can damage the delicate IC die surface.
A second type of transfer molding technique provides cushioning to the areas where the transfer mold clamps the IC die surface by providing a renewable compliant film that covers the surface of the transfer mold. A new film is placed over the surface of the transfer mold each time the transfer mold is used. Therefore, the transfer mold always presents a new clean compliant surface to each IC die surface on which it is used.
The compliant film is usually in the form a continuously fed tape. The transfer mold is placed over a portion of the film tape. Special pins and vacuum ports in the transfer mold must be provided to stretch the film, shape the film into the desired form, and hold the film in place against the contours of the surfaces of the mold cavity of the transfer mold. This is necessary to conform the film to the contours of the inner surface of the mold cavity at the time that the transfer mold process occurs. This process is referred to as the “film assisted molding” (“FAM”) technique. See, for example, U.S. Pat. No. 5,800,841. The FAM technique limits the type of materials that may be used because not all types of compliant material can be successfully stretched and formed using the FAM technique.
There is therefore a need in the art for a system and method for providing an improvement to the transfer molding process known as “film assisted molding.”